Cardiac rhythm management system with ultrasound for autocapture or other applications

ABSTRACT

A cardiac rhythm management system provides ultrasound autocapture capability for determining whether a stimulation has evoked a desired response from the heart, and for adjusting an energy of the stimulation based on the observed response from the heart. A first ultrasound element is disposed on a lead in the heart. A second ultrasound element is disposed elsewhere in the heart or in the implanted device. An autocapture determination circuit determines whether motion of the heart chamber indicates a contraction in response to the stimulation, and adjusts the stimulation energy to provide only that energy which is needed to obtain capture. This saves energy, prolonging the life of the implanted device, minimizing the risk and expense to patient associated with early explantation and replacement of the implanted device. Other applications include using ultrasound for (1) determining the strength of heart contractions (2) determining dissociation between electrical and mechanical heart activity, (3) determining the volume of the heart, (4) determining the origin of sensed intrinsic electrical heart activity signals, (5) recognizing particular arrhythmias (6) disrupting cell membranes for lowering stimulation thresholds, (7) controlling the delivery of a steroid, and (8) obtaining blood flow information.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a continuation of U.S. application Ser. No.09/950,276, filed on Sep. 10, 2001, which is a continuation of U.S.application Ser. No. 09/294,641, filed on Apr. 19, 1999, now issued asU.S. Pat. No. 6,298,269, the specifications of which are herebyincorporated by reference.

TECHNICAL FIELD

[0002] This invention relates generally to cardiac rhythm managementsystems and particularly, but not by way of limitation, to a cardiacrhythm management system with ultrasound autocapture capability fordetermining whether a stimulation has evoked a desired response from theheart.

BACKGROUND

[0003] When functioning properly, the human heart maintains its ownintrinsic rhythm, and is capable of pumping adequate blood throughoutthe body's circulatory system. However, some people have irregularcardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmiasresult in diminished blood circulation. One mode of treating cardiacarrhythmias uses drug therapy. Drugs are often effective at restoringnormal heart rhythms. However, drug therapy is not always effective fortreating arrhythmias of certain patients. For such patients, analternative mode of treatment is needed. One such alternative mode oftreatment includes the use of a cardiac rhythm management system. Suchsystems are often implanted in the patient and deliver therapy to theheart.

[0004] Cardiac rhythm management systems include, among other things,pacemakers, also referred to as pacers. Pacers deliver timed sequencesof low energy electrical stimuli, called pace pulses, to the heart, suchas via a transvenous leadwire or catheter (referred to as a “lead”)having one or more electrodes disposed in or about the heart. Heartcontractions are initiated in response to such pace pulses (this isreferred to as “capturing” the heart). By properly timing the deliveryof pace pulses, the heart can be induced to contract in proper rhythm,greatly improving its efficiency as a pump. Pacers are often used totreat patients with bradyarrhythmias, that is, hearts that beat tooslowly, or irregularly.

[0005] Cardiac rhythm management systems also include cardioverters ordefibrillators that are capable of delivering higher energy electricalstimuli to the heart. Defibrillators are often used to treat patientswith tachyarrhythmias, that is, hearts that beat too quickly. Suchtoo-fast heart rhythms also cause diminished blood circulation becausethe heart isn't allowed sufficient time to fill with blood beforecontracting to expel the blood. Such pumping by the heart isinefficient. A defibrillator is capable of delivering an high energyelectrical stimulus that is sometimes referred to as a defibrillationcountershock. The countershock interrupts the tachyarrhythmia, allowingthe heart to reestablish a normal rhythm for the efficient pumping ofblood. In addition to pacers, cardiac rhythm management systems alsoinclude, among other things, pacer/defibrillators that combine thefunctions of pacers and defibrillators, drug delivery devices, and anyother systems or devices for diagnosing or treating cardiac arrhythmias.

[0006] One problem faced by cardiac rhythm management systems isproviding therapy at appropriate energy levels. In pacers, for example,pacing stimulations must have sufficient energy to capture the heart,that is, to initiate a resulting heart contraction. Delivering too muchenergy, however, will shorten the life of the battery poweredimplantable device. This, in turn, results in performing an earliersurgical explantation and replacement procedure, with its attendantrisks and costs, both for the procedure and for the replacement device.Thus, there is a need to determine whether a cardiac rhythm managementsystem is providing therapy at appropriate energy levels.

SUMMARY

[0007] This document describes, among other things, a cardiac rhythmmanagement system with ultrasound autocapture capability for determiningwhether a stimulation has evoked a desired response from the heart, andfor adjusting an energy of the stimulation based on the observedresponse from the heart. An autocapture determination circuit determineswhether motion of the heart chamber indicates a contraction in responseto the stimulation, and adjusts the stimulation energy to provide onlythat energy which is needed to ensure reliable capture. This savesenergy, prolonging the life of the implanted device, minimizing the riskand expense to patient associated with early explantation andreplacement of the implanted device.

[0008] In one embodiment, the cardiac rhythm management system includesa lead. The lead includes a distal end and a proximal end. The distalend of the lead is adapted for being disposed in or about a heart. Thedistal end of the lead includes a first ultrasonic element. Anelectronics unit is coupled to the proximal end of the lead. Theelectronics unit includes an ultrasound driving circuit and a signalprocessing circuit that includes an autocapture determination circuit.In a first further embodiment, a second ultrasonic element is adaptedfor being disposed in a heart chamber different from the firstultrasonic element. In a second further embodiment, a second ultrasonicelement is in a case carrying the electronics unit.

[0009] This document also discloses using the ultrasound forapplications other than the ultrasound capability, including but notlimited to: (1) determining the strength of heart contractions, such asfrom the slope of a signal transduced from the ultrasound, (2)determining dissociation between electrical and mechanical heartactivity, (3) determining the volume of the heart, such as at differenttimes during the cardiac cycle, (4) determining the origin of sensedintrinsic electrical heart activity signals based at least in part oninformation obtained from the ultrasound, (5) recognizing particulararrhythmias based at least in part on information obtained from theultrasound, (6) delivering ultrasound for disrupting cell membranes forlowering stimulation thresholds, (7) using ultrasound to control thedelivery of a steroid, and (8) obtaining blood flow information based atleast in part on information obtained from the ultrasound. Other aspectsof the invention will be apparent on reading the following detaileddescription of the invention and viewing the drawings that form a partthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the drawings, like numerals describe substantially similarcomponents throughout the several views. Like numerals having differentletter suffixes represent different instances of substantially similarcomponents.

[0011]FIG. 1 is a schematic drawing illustrating generally oneembodiment of portions of a cardiac rhythm management system and anenvironment in which it is used.

[0012]FIG. 2 is a schematic drawing illustrating generally oneembodiment of portions of a lead and an implanted device.

[0013]FIG. 3 is a schematic drawing illustrating generally oneembodiment of portions of a lead, an implanted device, and circuitsincluded in the implanted device.

[0014]FIG. 4 is a schematic drawing, illustrating generally oneembodiment of transmission of ultrasound from the device to theultrasonic element disposed in the heart.

[0015]FIG. 5 is a schematic drawing, illustrating generally anembodiment including a first ultrasonic element in a first heart chamberand a second ultrasonic element disposed in another heart chamber.

[0016]FIG. 6 is a schematic drawing illustrating generally an embodimentincluding a first ultrasonic element in a first heart chamber, a secondultrasonic element in another heart chamber, and a third ultrasonicelement in the implanted device.

[0017]FIG. 7 is a schematic drawing illustrating generally an embodimentincluding a single ultrasonic element in a first heart chamber for bothproviding and receiving ultrasound energy.

[0018]FIG. 8 is a schematic drawing illustrating generally an embodimentproviding capabilities other than autocapture.

DETAILED DESCRIPTION

[0019] In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims and their equivalents. In thedrawings, like numerals describe substantially similar componentsthroughout the several views. Like numerals having different lettersuffixes represent different instances of substantially similarcomponents.

[0020] This document describes, among other things, a cardiac rhythmmanagement system with ultrasound autocapture capability for determiningwhether a stimulation has evoked a desired response from the heart, andfor adjusting an energy of the stimulation based on the observedresponse from the heart.

[0021]FIG. 1 is a schematic drawing illustrating generally, by way ofexample, but not by way of limitation, one embodiment of portions of acardiac rhythm management system 100 and an environment in which it isused. In FIG. 1, system 100 includes an implantable cardiac rhythmmanagement device 105, which is coupled by an intravascular endocardiallead 110 to a heart 115 of patient 120. System 100 also includes anexternal programmer 125 providing wireless communication with device 105using a telemetry device 130. Lead 110 includes a proximal end 135,which is coupled to device 105, and a distal end 140, which is coupledto one or more portions of heart 115.

[0022]FIG. 2 is a schematic drawing illustrating generally, by way ofexample, but not by way of limitation, one embodiment of portions oflead 110 and device 105, with distal end 140 of lead 110 being disposedin right ventricle 200 of heart 115. Right atrium 205, left atrium 210,and left ventricle 215 are also illustrated. Distal end 140 of lead 110includes at least one electrode, such as pacing tip electrode 220 andpacing ring electrode 225, for providing bipolar pacing stimulations toheart 115. Tip electrode 220 is coupled to device 105 by a first wirecarried in lead 110. Ring electrode 225 is coupled to device 105 by asecond wire carried in lead 110. Distal end 140 of lead 110 alsoincludes a piezoelectric first ultrasonic element 230, located near ringelectrode 225 and tip electrode 220, for transmitting ultrasound,receiving ultrasound, or both transmitting and receiving ultrasound.Device 105 includes a piezoelectric second ultrasonic element 235 fortransmitting ultrasound, receiving ultrasound, or both transmitting andreceiving ultrasound. In one embodiment, second ultrasonic element 235is attached within a hermetically sealed case that encloses theelectronic and other components of device 105. In another embodiment,second ultrasonic element 235 is included within a header portionextending from the hermetically sealed case, as illustrated in FIG. 6.

[0023]FIG. 3 is a schematic drawing illustrating generally, by way ofexample, but not by way of limitation, portions of heart 115, includingfirst ultrasound element 230 and at least one electrode 220 disposedtherein, and device 105, including second ultrasound element 235. Inthis embodiment, an electrical driving signal at ultrasonic frequencies(e.g., 1-100 microsecond pulses of a 10 MHZ ultrasound signal) isprovided by driver 300, in device 105, through conductors in lead 110 tofirst ultrasound element 230. Transduced from this signal, firstultrasound element 230 emits ultrasound energy within heart 115. Thisultrasound energy is received by second ultrasound element 235 in device105, which transduces it into an electrical signal that is provided, atnode 305, to signal processor 310. Signal processor 310 includesamplification, demodulation, filter, analog-to-digital (A/D) conversion,digital-to-analog (D/A) conversion, memory, and other circuits forextracting and storing information obtained from the transducedultrasound signal from second ultrasound element 235. Portions of thesecircuits may be implemented as one or more sequences of instructionscarried out on a microprocessor or other microcontroller.

[0024] The signal at node 305 includes information about the movement ofthe heart chamber in which first ultrasound element 230 is disposed.Therapy circuit 315 delivers a pacing pulse stimulation via lead 110 toelectrode 220 in right ventricle 200 (for example) of heart 115. Suchpacing stimuli are usually delivered at a time when the particular heartchamber is in a relaxed passive state and is being filled with blood. Ifthe delivered pacing stimulus captures the heart, myocardial tissue nearpacing site of electrode 220 begins to contract. If the delivered pacingstimulus does not capture the heart, such tissue does not begin tocontract.

[0025] In one embodiment, system 100 uses a measurement of the transittime of the ultrasound energy between first ultrasound element 230,which is disposed in right ventricle 200, and second ultrasound element235, in device 105 located at a reference point elsewhere in patient 120outside heart 115. The transit time of the ultrasound energy provides anindication of the physical distance between first ultrasound element 230and second ultrasound element 235. Variations in this distance representmovement of right ventricle 200, in which first ultrasound element 230is disposed, relative to device 105. Such movement of right ventricle200 results from contractions caused by the pacing stimulations orintrinsic electrical heart activity. The presence or absence of suchmovement during an appropriate time period following the pacingstimulation indicates a resulting capture and no capture, respectively.

[0026] By monitoring movement over that portion of the cardiac cyclefollowing the pacing stimulation, system 100 determines whether thepacing stimulation captured the heart. In one embodiment, the movementobserved (during an appropriate time window initiated by the pacingstimulation) is compared to a threshold value, which corresponds to themovement expected even in the absence of an evoked contraction. If themovement observed exceeds this threshold value, then system 100indicates that capture was successfully obtained. If, upon expiration ofthe appropriate time period, system 100 determines that the heart wasnot captured, then system 100 issues a second pacing stimulation, ofsufficiently large energy to obtain capture. In another embodiment,movement is monitored over a plurality of cardiac cycles to determinewhether, over a period of time, the pacing stimulations capture theheart. For each pacing stimulation, if no capture is indicated uponexpiration of the time window triggered by the pacing stimulation, thena second pacing stimulation of sufficient energy to obtain capture isdelivered.

[0027] One autocapture method of determining the appropriate energylevel starts with the delivery of a high energy pacing stimulation tothe heart. The energy is reduced during subsequent pacing stimulations(e.g., by reducing pacing amplitude or pulse duration), until themovement observed indicates that capture was not obtained. System 100then increases the energy back to a level known to be adequate to obtaincapture. This autocapture determination can be made intermittently or,alternatively, may represent a particular mode that can be set tooperate continuously.

[0028] In another embodiment, system 100 uses a Doppler shiftmeasurement of movement of the right ventricle 200. In this embodiment,the change in frequency of the ultrasound signals transmitted by thefirst ultrasound element 230 (and received by second ultrasound element235) is frequency demodulated to represent movement of right ventricle200 as it contracts and expands. A higher frequency signal representsmotion of the first ultrasound element 230, in right ventricle 200,toward device 105. A lower frequency signal represents motion of thefirst ultrasound element 230, in right ventricle 200, away from device105. In a further embodiment, both transit time and Doppler shiftmeasurements (or other like measurements) are used.

[0029] System 100 includes an autocapture determination module (e.g., asequence of instructions carried out on the microcontroller in signalprocessor 310) that determines whether motion of the heart chamberindicates a contraction in response to the delivered stimulation. Theautocapture determination module adjusts the stimulation energy (eitherthe amplitude of the pacing voltage pulse or the time period, i.e.,pulsewidth, during which the pacing stimulation is being delivered) toprovide only that stimulation energy which is needed to obtain capture.As a result, the energy delivered by therapy circuit 315 is optimizedsuch that, over many such cardiac cycles, pacing stimulations aredelivered at optimized reduced energies. This, in turn, prolongs thelife of power source 320 (e.g., a battery) and the implanted device 105,minimizing the risk and expense to patient 120 associated with earlyexplantation and replacement of device 105.

[0030]FIG. 4 is a schematic drawing, similar to FIG. 3, illustratinggenerally, by way of example, but not by way of limitation, anotherembodiment of portions of system 100 in which driver 300 is coupled tosecond ultrasound element 235 for emitting ultrasound energy received byfirst ultrasound element 230 in heart 115, where it is transduced intoan electrical signal that is provided, at node 305, to signal processor310. This embodiment also contemplates one or more of transit time,Doppler shift, or other like measurements of the motion of firstultrasound element 230 to detect whether a stimulation from therapycircuit 315 obtains a resulting contraction of heart 115.

[0031]FIG. 5 is a schematic drawing, including aspects that are similarto those described with respect to FIG. 2, illustrating generally, byway of example, but not by way of limitation, another embodiment ofportions of system 100 in which device 105 is also coupled to a secondheart chamber, i.e., right atrium 205, through lead 110B. Second lead110B also includes tip electrode 220B, ring electrode 225B, and secondultrasonic element 230B. In this embodiment, a stimulation is deliveredto right ventricle 200 and its movement is detected by providingultrasound at first ultrasound element 230A and receiving the ultrasoundat second ultrasound element 230B (or vice versa) to determine whetherthe stimulation obtained a resulting contraction of right ventricle 200.Similarly, a stimulation is delivered to right atrium 205 and itsmovement is detected by providing ultrasound at second ultrasoundelement 230B and receiving the ultrasound at first ultrasound element230A (or vice versa) to determine whether the stimulation obtained aresulting contraction of right atrium 205.

[0032]FIG. 6 is a schematic drawing, similar to FIGS. 2 and 5,illustrating generally, by way of example, but not by way of limitation,another embodiment of portions of system 100 in which device 105 alsoincludes a third ultrasonic element 235. This embodiment alsoillustrates, by way of example, locating third ultrasonic element 235 ina header portion extending from the hermetically sealed case.Alternatively, third ultrasonic element 235 is located in thehermetically sealed case, as illustrated in FIG. 2. In th embodiment ofFIG. 6, a stimulation is delivered to right ventricle 200, and itsmovement is detected by providing ultrasound at first ultrasound element230A and receiving the ultrasound at third ultrasound element 235 (orvice versa) to determine whether the stimulation obtained a resultingcontraction of right ventricle 200. Similarly, a stimulation isdelivered to right atrium 205, and its movement is detected by providingultrasound at second ultrasound element 230B and receiving theultrasound at third ultrasound element 235 (or vice versa) to determinewhether the stimulation obtained a resulting contraction of right atrium205.

[0033]FIG. 7 is a schematic drawing illustrating another embodiment ofportions of system 100 in which a single ultrasound element, such asfirst ultrasound element 230, both provides and receives ultrasoundenergy. In this embodiment, first ultrasound element 230 is coupled todriver 300. Ultrasound element 230 converts electrical energy providedby driver 300 into an ultrasound energy pulse that is delivered to heart115. Then, driver 300 is turned off, and ultrasound element 230 receivesreflected ultrasound energy, such as from any acoustic boundary, forexample, the interior wall of heart 115. The ultrasound energy reflectedfrom the heart wall and received by ultrasound element 230 is transducedby ultrasound element 230 into an electrical signal that is provided, atnode/bus 305 to signal processor 310.

[0034] In one embodiment, variations in the transit time of the receivedultrasound energy that is reflected from the heart wall provides anindication of the movement of the heart due to a contraction. In anotherembodiment, variations in the Doppler shift of the received ultrasoundenergy that is reflected from the heart wall provides an indication ofthe movement of the heart due to a contraction. In a further embodiment,both transit time and Doppler shift measurements (or other likemeasurements) are used. Using the techniques described above, system 100includes an ultrasound autocapture method that determines whether aparticular pacing stimulation captured the heart based on the ultrasoundindication of movement of the heart during a time window following thepacing stimulation.

Other Applications

[0035] Although it is described above primarily with respect to usingultrasound for providing autocapture capability, system 100 includesalso other uses, as illustrated generally, by way of example, but not byway of limitation, in the schematic drawing of FIG. 8. In a firstexample, the ultrasound signal provides information about mechanicalheart contractions. The ultrasound signal is transduced and processed toobtain an electrical signal that includes information about the heartcontractions. From the slope other characteristics of this electricalsignal, a slope detection module 800 determines the strength of theheart contraction. In one embodiment, the slope detection module 800 isimplemented as a sequence of instructions executed by signal processor310. For example, stronger contractions occur during a shorter period oftime, corresponding to a larger slope in the ultrasonically transducedmechanical heart contraction signal. This information is used fordiagnostic purposes (e.g., communicated to programmer 125) or to adjusttherapy, such as by adjusting rate, timing, or energy to maximize thestrength of the heart contraction.

[0036] In a second example, the ultrasound signal provides informationabout mechanical heart contractions. Intrinsic electrical heart activitysignals obtained, at sensing circuit 815, from the lead electrodes.These intrinsic electrical heart activity signals are also referred toas electrogram signals. The electrogram signals provide informationabout the electrical heart activity which causes the heart contractions.The ultrasound and the electrogram signals are compared by adissociation detection module 805 implemented as a sequence ofinstructions executed in signal processor 310 to determine whether, andto what degree, there exists any dissociation between the time ofintrinsic electrical heart activity and the corresponding resultingmechanical contractions of the heart. This information is used fordiagnostic purposes (e.g., communicated to programmer 125) or to adjusttherapy, such as by adjusting rate, timing, or energy delivered tooptimize the relationship between intrinsic electrical heart activityand the mechanical contractions of the heart.

[0037] In a third example, the ultrasound signal provides informationabout mechanical heart contractions. The ultrasound signal is transducedand processed to obtain an electrical signal that includes informationabout the heart contractions. From the amplitude or othercharacteristics of this signal, the filling and end diastolic volume ofthe heart is determined. In one example, a amplitude detection module810, such as a peak detection module implemented as a sequence ofinstructions executed in signal processor 310, is used to obtain anindirect measure of the volume of the heart using amplitudemeasurements. This information is used for diagnostic purposes (e.g.,communicated to programmer 125). For example, increases in heart volumemay indicate degeneration toward congestive heart failure (CHF).Alternatively, the ultrasonic signal information is used to adjusttherapy, such as by adjusting rate, timing, or energy delivered tomaximize cardiac output or efficiency.

[0038] In a fourth example, the ultrasound signal provides informationabout mechanical heart contractions. Electrogram signals, from sensingcircuit 815, provide information about the electrical heart activitythat causes the heart contractions. Such electrogram signals may includefar-field sensing of electrical heart activity associated with heartchambers other than the heart chamber in which the electrode providingthe signal is located. The ultrasonically obtained heart contractioninformation is used to augment the electrical heart activity informationfrom sensing circuit 815 for better determining the origin of the sensedelectrical heart activity (e.g., near-field or far-field) so thattherapy can be provided based on a more accurate determination of heartactivity. Conversely, the ultrasound signal may include componentsassociated with heart chambers other than the heart chamber associatedwith the ultrasound element. In such a case, the electrogram informationmay alternatively be used to better determine the origin of theultrasonically-indicated mechanical heart contraction signal so thattherapy can be provided based on a more accurate determination of heartactivity.

[0039] In a fifth example, the ultrasound signal provides informationabout mechanical heart contractions. Electrogram signals from sensingcircuit 815 provide information about the electrical heart activity thatcauses the heart contractions. Certain arrhythmias, such as sinustachycardia and ventricular tachycardia are difficult to distinguishfrom each other based on electrogram signals. Moreover, defibrillationcountershock therapy may be appropriate for a ventricular tachycardia,but unnecessary for a sinus tachycardia. However, the mechanical heartcontraction signal characteristics are different for sinus tachycardiaas compared to ventricular tachycardia. In one embodiment, system 100discriminates between sinus tachycardia, ventricular tachycardia, andother arrhythmias based on the mechanical heart contraction signalobtained from one or more ultrasound elements disposed in one or more ofright ventricle 200, right atrium 205, or other heart chamber.Anti-tachyarrhythmia therapy is tailored to the particular type ofarrhythmia detected. In one embodiment, defibrillation countershocks aredelivered for ventricular tachyarrhythmias, but are not delivered forsinus tachyarrhythmias. System 100 saves energy by avoiding theinappropriate delivery of defibrillation countershocks for sinustachyarrhythmias. This minimizes the risk and expense to patient 120associated with early explantation and replacement of device 105.Avoiding inappropriate delivery of defibrillation countershocks alsoavoids irritating the heart and causing further arrhythmias. Even moreimportantly, system 100 decreases the risk of actually causing aventricular tachyarrhythmia, such as life-threatening ventricularfibrillation, by inappropriately delivering a shock.

[0040] In a sixth example, the signal processor 310 controls operationof endocardial ultrasonic element 225 such that it delivers localizedultrasound energy pulses to at least partially disrupt cell membranesnear the pacing tip electrode 220. This is expected to lower the pacingthreshold energy required to capture the heart 115 by decreasing theelectrical resistance of cell membranes near the pacing electrode. Oneor more pacing pulses are then delivered at reduced energies, such as byusing the autocapture techniques described above. This prolongs theusable life of implanted device 105. In one embodiment, delivery of theultrasound energy pulse is synchronized to delivery of a pacing pulse.

[0041] In a seventh example, the signal processor 310 controls operationof endocardial ultrasonic element 225 such that it modulates the releaseof a steroid from a nearby steroid eluting element, such as a polymericsteroid eluting matrix. In one example, a steroid eluting matrix isassociated with nearby pacing electrode 220 to reduce the formation ofscar tissue after implant so that lower pacing threshold energies areobtained. In one embodiment, device 105 delivers ultrasound energy toenhance the rate of steroid delivery. Because device 105 can accuratelycontrol the timing, duration, and energy of the ultrasound, increasedcontrol is obtained over the release of the steroid, which is ordinarilygoverned primarily by the physical characteristics of the steroideluting matrix. In one embodiment, device 105 is programmably operatedto release the steroid either more immediately after implant or after adesired time delay.

[0042] In an eighth example, the endocardial ultrasonic element 225 isused to measure blood flow in the heart chamber in which it is disposed,using transit time, Doppler shift, or other measurement techniques. Thisblood flow information is used for diagnostic purposes or to adjusttherapy, such as by adjusting rate, timing, or energy delivered tomaximize cardiac output based on the blood flow measurements.

CONCLUSION

[0043] The above-described system provides, among other things, acardiac rhythm management system with ultrasound autocapture capabilityfor determining whether a stimulation has evoked a desired response fromthe heart, and for adjusting an energy of the stimulation based on theobserved response from the heart. An autocapture determination circuitdetermines whether motion of the heart chamber indicates a contractionin response to the stimulation, and adjusts the stimulation energy toprovide only that energy which is needed to obtain capture. This savesenergy, prolonging the life of the implanted device, minimizing the riskand expense to a patient associated with early explantation andreplacement of the implanted device.

[0044] Although the system was described above primarily with respect todisposing an ultrasound element in the right ventricle for autocapturedetermination, it is understood that the system also includesapplication to ultrasonic autocapture determination in other heartchambers, and to the use of ultrasound for applications other thanautocapture, some of which are described above.

[0045] It is to be understood that the above description is intended tobe illustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A cardiac rhythm management system comprising: alead, comprising a distal end and a proximal end, the distal end of thelead adapted for being disposed in or about a heart, the distal end ofthe lead comprising a first ultrasonic element; and an electronics unitcoupled to the proximal end of the lead, the electronics unitcomprising: an ultrasound driving circuit; and a signal processingcircuit that processes a signal that is based on ultrasound, and inwhich the signal processing circuit includes an autocapturedetermination module that determines, using information from the signalthat is based on ultrasound, whether a stimulus triggered a resultingheart contraction.
 2. The system of claim 1, further comprising: anelectrode adapted for being associated with a portion of a heart; and asensing circuit for detecting intrinsic electrical heart activity basedon at least one electrogram signal received from the electrode.
 3. Thesystem of claim 2, in which the signal processing circuit detectsoccurrences of heart activity using both the ultrasound and theelectrogram signal.
 4. The system of claim 1, further comprising aprogrammer remote from the electronics unit.
 5. A cardiac rhythmmanagement system comprising: an electronics unit, sized and shaped topermit implantation in a human subject, the electronics unit comprising:an ultrasound driving circuit; and means for determining, usinginformation from returned ultrasound, whether an electrical stimulustriggered a resulting heart contraction.
 6. The system of claim 5,further comprising a lead coupled to the electronics unit, the leadcomprising a distal end and a proximal end, the distal end of the leadadapted for being disposed in or about a heart, the distal end of thelead comprising a first ultrasonic element.
 7. The system of claim 5,further comprising a lead coupled to the electronics unit, the leadcomprising a distal end and a proximal end, the distal end of the leadadapted for being disposed in or about a heart, the distal end of thelead comprising an electrode for delivering the electrical stimulus. 8 Acardiac rhythm management system comprising: an electronics unit, sizedand shaped to permit implantation into a human subject, the electronicsunit comprising: an ultrasound driving circuit; and a signal processingcircuit that processes a signal that is based on ultrasound, and inwhich the signal processing circuit includes a slope detection modulethat provides, using information from the signal that is based onultrasound, an indication of a strength of a mechanical heartcontraction.
 9. The system of claim 8, further comprising a lead,coupled to the electronics unit, the lead comprising a distal end and aproximal end, the distal end of the lead adapted for being disposed inor about a heart, the distal end of the lead comprising a firstultrasonic element.
 10. A cardiac rhythm management system comprising:an electronics unit, sized and shaped to permit implantation into ahuman subject, the electronics unit comprising: an ultrasound drivingcircuit; and means for determining a strength of a mechanical heartcontraction using information from a signal that is based on ultrasound.11. A cardiac rhythm management system comprising: an electronics unitcoupled to the proximal end of the lead, the electronics unit sized andshaped to permit implantation into a human subject, the electronics unitcomprising: an ultrasound driving circuit; and a signal processingcircuit that processes a signal that is based on ultrasound, and inwhich the signal processing circuit includes an amplitude detectionmodule that detects an amplitude of the signal that is based onultrasound.
 12. The system of claim 11, in which the amplitude detectionmodule provides an indication of the volume of a mechanical heartcontraction using the detected amplitude of the signal that is based onultrasound.
 13. The system of claim 11, further comprising a lead,comprising a distal end and a proximal end, the distal end of the leadadapted for being disposed in or about a heart, the distal end of thelead comprising a first ultrasonic element.
 14. A cardiac rhythmmanagement system comprising: an electronics unit, sized and shaped topermit implantation into a human subject, the electronics unitconfigured to be coupled to an electrode associated with a heart and toan ultrasonic element associated with the heart, the electronics unitcomprising: an ultrasound driving circuit; a signal processing circuitthat processes a signal that is based on returned ultrasound; a sensingcircuit to detect intrinsic electrical heart activity using at least oneelectrogram signal received from the electrode, and in which the signalprocessing circuit is configured to detect a dissociation between (1) anoccurrence of heart activity detected using the signal that is based onreturned ultrasound and (2) an occurrence of heart activity detectedusing the at least one electrogram signal.
 15. The system of claim 14,further comprising a lead comprising a distal end and a proximal end,the distal end of the lead adapted for being disposed in or about aheart, the distal end of the lead comprising the ultrasonic elementassociated with the heart.
 16. The system of claim 14, furthercomprising a lead comprising a distal end and a proximal end, the distalend of the lead adapted for being disposed in or about a heart, thedistal end of the lead comprising the electrode associated with theheart.
 17. The system of claim 14, further comprising a lead comprisinga distal end and a proximal end, the distal end of the lead adapted forbeing disposed in or about a heart, the distal end of the leadcomprising the electrode associated with the heart and the ultrasonicelement associated with the heart.
 18. A cardiac rhythm managementsystem comprising: an electronics unit, sized and shaped to permitimplantation into a human subject, the electronics unit configured to becoupled to an electrode associated with a heart and to an ultrasonicelement associated with the heart, the electronics unit comprising: anultrasound driving circuit; a sensing circuit to detect intrinsicelectrical heart activity using at least one electrogram signal receivedfrom the electrode; and means, coupled to the sensing circuit, fordetecting any dissociation between (1) an occurrence of heart activitydetected using returned ultrasound and (2) an occurrence of heartactivity detected using the at least one electrogram signal.
 19. Thesystem of claim 18, further comprising a lead comprising a distal endand a proximal end, the distal end of the lead adapted for beingdisposed in or about a heart, the distal end of the lead comprising theultrasonic element associated with the heart.
 20. The system of claim18, further comprising a lead comprising a distal end and a proximalend, the distal end of the lead adapted for being disposed in or about aheart, the distal end of the lead comprising the electrode associatedwith the heart.
 21. The system of claim 18, further comprising a leadcomprising a distal end and a proximal end, the distal end of the leadadapted for being disposed in or about a heart, the distal end of thelead comprising the electrode associated with the heart and theultrasonic element associated with the heart.